Autotrophs
and primary production--
Section 6 introduced the autotrophs and described some of the factors
influencing them. The main points to be re-emphasized here are:
• Periphyton (or biofilm) – especially diatoms and cyanobacteria – is the main source of primary production in shallow stony rivers, but riparian vegetation causes shading that can limit photosynthesis, and nutrients may be in short supply.
• The surface of wider rivers receives ample light because of the reduced shading effect of riparian vegetation. However, in deep or turbid sections, light penetration may be insufficient to sustain periphyton growth.
• Macrophyte production is significant in lowland reaches only, and rooted macrophytes may be limited by light penetration in turbid water.
• Phytoplankton is limited by turbidity (i.e., water clarity) in large rivers and sometimes by nutrients also, but can be important in some sites – especially on floodplains.
• Wet-season rains tend to reduce autotroph biomass, and this is likely to result in a decline in primary production.
The limiting effects of light (through turbidity or
shading) and nutrients vary from place to place and time to time, thus
it is difficult to generalize about the amounts of primary production
in rivers. There is considerable variation within and between river systems,
and there is no clear large-scale trend in the magnitude of primary production.
Where the waters are not turbid, production in shaded rivers is only a
fraction of that of open-canopy rivers of equivalent size, and production
in floodplain reaches can be considerably higher than upstream. Macrophytes
contribute to floodplain production both by the elaboration of biomass
and by providing a surface on which periphyton can grow.
Around 90% of the biomass of terrestrial plants enters the detritus food
chain, but animals eat much of the periphyton and phytoplankton. Aquatic
macrophytes, like terrestrial plants, are generally eaten only after they
have died.
Particulate and dissolved
organic matter--
Most allochthonous material enters rivers in the headwaters, or
originates in flooded forests on the lower course. At its point of entry,
much of this material is in its original form. It may range from large
woody debris (tree trunks and branches) to smaller parts of plants (leaves
and twigs), as well as insects and other small invertebrates. This coarse
particulate organic material (CPOM: >1 mm in diameter) is gradually
fragmented into fine particulate organic material (FPOM: <1 mm in diameter)
then ultra-fine particulate organic matter (UFPOM: <75 microns in diameter)
as it is processed by aquatic organisms (see next section). Thus, there
is as a gradual reduction in the size of the particles as they are carried
downstream.
Dissolved organic matter (DOM; <0.5 microns in diameter) is an important
component of the organic material in rivers. Much of it enters via subsurface
drainage and originates from terrestrial decomposition processes; other
sources are detrital leaching, and exudates and excreta from aquatic organisms.
DOM tends to increase in concentration downstream. The highest levels
occur in blackwater rivers, especially those draining peatswamps, which
are rich in humic substances that colour the water. DOM is taken up directly
by microorganisms – especially bacteria – in biofilms (see Section 6),
and can be flocculated into UFPOM or FPOM by mechanical forces such as
turbulence whereupon they become available to animals.
In large rivers, DOM is absorbed onto fine particles of sediment to form
organic aggregates. This material should not be equated with detritus,
as its composition is difficult. This material is an important part of
the diet of a special group of fishes that have especially long guts allowing
them to digest these aggregates.
